The present invention relates to electronic power metering utilizing digital signal processing and, more particularly, to simplified means to improve the accuracy of such meters by compensating for undesired differences or errors in the phase of the voltage and current signals.
Electronic watthour meters are disclosed, for example, in U.S. Pat. No. 4,535,287 Milkovic; U.S. Pat. No. 4,556,843 Milkovic and Bogacki; U.S. Pat. No. 4,682,102 Milkovic; and U.S. Pat. No. 4,761,605 Jochum; all of which are assigned to the same assignee as the present invention. In the present invention, an electronic watthour meter converts the analog signals proportional to current and voltage in the circuit being metered to digital signals for digital signal processing. The current signals are first converted to voltage signals by a current to voltage converter for compatibility with the analog to digital converters which provide digital signals based on samples of the current and voltage input signals. The conversion of such signals to binary to binary form is based on the amplitude of the current and voltage signals. The digital signals can then be readily multiplied and the product of the multiplication is added to an accumulator which generates pulses proportional to the power use on the power lines being metered. The accuracy of such electronic watthour meters requires that the voltage and current signals be in the correct phase relationship before each is provided to its analog to digital converter. That is, the phase relationship should accurately represent that in the power lines. However, the voltage and current scaling and isolation is accomplished through circuitry including instrument transformers and other circuit elements which may introduce phase differences or errors between them.
In order to obtain the required accuracy, and to compensate for manufacturing tolerances, errors in the instrument transformers and changes in circuit elements with time and/or environmental exposure, it becomes necessary to provide phase adjusting or compensating means for the electronic watthour meter. However, it is important that such conpensating means be simple, inexpensive, stable, and reliable. Power meter users, such as power companies and electrical utilities are used to simple adjustment in present rotating element magnetic watthour meters which require tools no more complex than a screwdriver. The phase adjustment requires effectively compensating for, or shifting the phase of, the voltage signal from the secondary winding of the voltage transformer relative to the current signal from the current sensor the amount of their error phase difference. However, in three phase power meters there are separate current and voltage isolation and scaling transformers for each of the three phases. Conventional phase shifters utilizing a variable resistor and/or variable capacitor for each of the three phases would increase the material cost and complexity (and hence reduce reliability), and would require additional cost and complexity for a technician to perform each adjustment individually, particularly in the field when recalibration may be undertaken by a customer. Moreover, variable RC network phase shifting is not applicable to multiplexed signals in three phase electronic power meters.